The placenta in the integrated physiology of fetal volume control.

Almost all water that enters the conceptus of the sheep enters via the placenta. The forces that drive water are hydrostatic and osmotic. The placental channels that allow water to cross into the fetus have not been identified by microanatomic means. Although an "equivalent pore" system can account for the diffusional entry of small hydrophilic solutes, it can be calculated that the filtration coefficient of this system is too small to account for the demonstrated trans-placental water flows. It is possible that a second much less numerous system of large pores permits the flow of water, but that is by no means certain. The placenta does not control the amount of water that enters the conceptus; nor does any other single fetal structure. And water entry is not dependent on the volume of water already present. However, the combined physiological properties of the fetal heart, kidneys, somatic tissues and placenta constitute a consistent explanation of fetal water volume control.